Phosphorus which occupies about 1% of people's weight is an element which constitutes DNA (deoxyribonucleic acid) which is a main body of a gene, ATP (adenosine triphosphate) which is required for energy metabolism and hydroxyapatite which is a main substance of a bone or a tooth. Phosphorus is an essential mineral in order for human beings to maintain life activity. Phosphorus is extracted from rock phosphate. It is said that an amount of deposits of phosphorus which can be mined are about 7 billion ton worldwide and it will be drained in about 100 years if an amount of consumption of phosphorus will increase at a rate of 2% every year from now on.
On the other hand, eventually phosphorus incorporated into agricultural products comes to be contained in wastewater via a human body, livestock, etc. The wastewater flowed into a closed water area and phosphorus has caused an environmental problem of eutrophication.
75% of phosphorus used in Japan is fixed as insoluble phosphorus in soil. However, about 10% of phosphorus is discharged via sewage or the wastewater of a factory, etc. in the water area. For this reason, recovering and recycling phosphorus from the wastewater becomes one of measures against the resource drain and the eutrophication. As techniques of recovering phosphorus, various methods, such as HAP method and MAP method which recovers phosphorus from the wastewater by chemical precipitation, and an elution method which elutes and recovers phosphorus from sludge incineration ashes using an acid and an alkali, etc. have so far been proposed.
The HAP method is a method of recovering phosphorus using a crystallization phenomenon (refer to, for example, the Japanese patent application public No. 2004-261640). In the HAP method, after Ca2+ and OH− were added to solution containing phosphorus and the solution was made into a supersaturation state (metastable region), the solution is led to a dephosphorus tank. The solution contacts dephosphorus material (rock phosphate, bone black, calcium silicate hydration liquid, etc.) in the tank, the hydroxyapatite (HAP:Ca10(OH2)(PO4)6) crystal precipitates on a filter material surface and phosphorus in a liquid is removed.
The MAP method is a method of recovering phosphorus using a crystallization phenomenon for a liquid containing phosphorus and ammonium (refer to, for example, the Japanese patent application public No. 2007-117948). In the MAP method, Mg2+ is added to object wastewater and MAP (MgNH4PO4) is generated in a weak alkali region (it grows up to be granulation which is 2-3 mm). An acid and alkaline elution method is a technique in which phosphorus in incineration ashes is eluted with the acid or the alkali and phosphate is extracted by solid liquid separation (refer to, for example, the Japanese patent application publication No. Hei 07-251141).
However, in the HAP method and the MAP method, in order to grow up HAP grains and MAP grains into a grain size which can be separated and recovered, sufficient reaction time is required. For this reason, a reaction tank of a large volume (superficial velocity=about 10 h−1 or less) is needed, and a large-scale equipment occupation area is needed. Therefore, equipment requires expense and it also becomes difficult to add the equipment to existing wastewater treatment facility. In addition, since a pH adjuster is added as pretreatment and Ca ion or Mg ion is added, chemicals cost is required and phosphorus recovery cost (running cost) increases. Furthermore, the wastewater after phosphorus recovery is changed in water quality owing to an additive etc. For this reason, when the wastewater after phosphorus recovery is returned to the existing wastewater treatment facility and is treated further, it is considered that water quality adjustment treatment is also needed before.
In the acid and alkaline elution method, since processing time becomes long, equipment becomes large-scale. Furthermore, a treatment facility for incineration ashes residue and the acid and the alkali is needed. In the acid elution, there is a possibility that heavy metals other than phosphorus may mix.